eq2000_to_gal_r

gdt.missions.fermi.gbm.localization.dol.legacy_functions.eq2000_to_gal_r(ra2000, dec2000)[source]

Function to convert from equatorial (ra, dec) coordinates in J2000 epoch to Galactic coordinates (lii, bii)

Parameters:

  • ra2000 (float32) – J2000 right ascension in radians

  • dec2000 (float32) – J2000 declination in radians

Returns:

  • lii (float32) – Galactic longitude in radians

  • bii (float32) – Galactic latitude in radians

  • Version V1.1

  • Subroutine to convert equatorial coordinates, RA_2000 & DEC_2000

  • (epoch 2000), to galactic coordinates, Lii and Bii.

  • All arguments are in RADIANS.

  • Michael S. Briggs, 16 July 1992, UAH / MSF! ES-62.

  • Method

  • Step 1 (Equatorial (epoch 2000) –> Equatorial (epoch 1950),)

  • Step 2 (Equatorial (epoch 1950) –> Galactic.)

  • These two steps are required because GRO uses epoch 2000 equatorial

  • coordinates and transformations from equatorial to galactic are

  • for epoch 1950 equatorial coordinates.

  • In essence, the galactic system is defined by the location of the

  • galactic center and galactic North pole, and these locations are defined

  • by the values they were assigned in 1950 coordinates. See J. Meus.

  • Step 1, precession from epoch 2000 to 1950, is somewhat simplified

  • from the procedure given in The Astronomical Almanac. I have left

  • out corrections which are probably unnecessary for BATSE’s position

  • accuracy, e.g. I have left out the E-terms, which take care of the

  • abberation effects due to the ellipticty of the earth’s orbit.

  • Step 1 is based upon p. B43 of the 1991 The Astronomical Almanac.

  • Step 2 is baed upon a rotation matrix created based upon the definition

  • of galactic coordinates. See subroutine GET_ETOG_MATRIX.

  • References

  • Astronomical Algorithms, J. Meeus, pp. 89 & 90, pp. 123-130 (esp.

  • pp. 129 & 130).

  • Astrophysical Formulae, 2nd. ed., K. R. Lang, pp. 498 & 504.

  • Practical Astronomy with your !alculator, 3rd ed., P. Duffett-Smith,

  • pp. 43 & 50.

  • Third Reference !atalogue of Bright Galaxies, de Vancouleurs et al.,

  • vol. 1, p. 11.

  • Third Reference !atalogue of Bright Galaxies, de Vancouleurs et al.,

  • vol. 2 & 3.

  • The Astronomical Almanac for the year 1991, p. B43.

  • Tested as follows – The Third Reference !atalogue of Bright Galaxies, volumes 2 & 3,

  • list the coordinates of many galaxies. The coordinates listed

  • include RA & DEC epoch 2000, RA & DEC epoch 1950, and Lii and Bii,

  • hence these volumes may serve as a Rosetta Stone for the testing

  • of coordinate systems. The volumes list RA to 0.1 sec of time and

  • DEC to 1 sec of arc. They list Lii and Bii both to 0.01 degrees. – Running this subroutine with input the RA and DEC of randomly

  • selected galaxies from these volumes, the output Lii and Bii were

  • always found to exactly agree with the numbers in the volumes.

  • Hence this subroutine is shown to be accurate to at least about

  • 0.01 degrees.

  • Further testing (Mark Finger wrote a program for the same purpose,)

  • known as J2000_TO_GALII. The programs were compared using 5000

  • points randomly located on the sphere. The maximum discrepancy

  • was 8 milli-arc seconds!

  • Input arguments

  • REAL*4 RA_2000 ! right ascension, RADIANS

  • REAL*4 DEC_2000 ! declination, RADIANS

  • Output arguments

  • REAL*4 LII ! galactic longitude, RADIANS

  • REAL*4 BII ! galactic latitude, RADIANS

  • Matrix to convert equatorial (epoch 2000) to equatorial (epoch 1950).

  • Remember that FORTRAN stores matrices backwards. Taken from page B43

  • of The Astronomical Almanac.